The myocardium is a continually working aerobic organ. Consequently it is essential for normal myocardial function, that continual supply of oxygen is maintained to ensure a proper balance between myocardial oxygen demands and myocardial oxygen supply. Thus, oxygen balance must be maintained during a variety of physiologic stresses, e.g., exercise, excitement, to prevent myocardial ischemia and dysfunction. Oxygen balance is intimately dependent on myocardial blood flow, which is determined by physical (inertia, viscosity, pressure contraction) and reactive (resistance, capacitance) components of the coronary circulation. Although there has been extensive definition of these parameters, for the entire coronary circulation as a single compartment, there have been a paucity of studies dealing with potential compartmental hierarchies of the reactive components as "fine tuning" mechanisms responsible for modulation of myocardial perfusion. The overall goal of the proposal is to define these reactive components of the coronary system and the potential functional hierarchy of the components. To accomplish these goals, I used a computer-controlled system which compensates for cardiac motion to enable microvascular pressure and diameter measurements in the beating heart. This computer controlled device can move a micropipette in three dimensions in concert with a moving microvessel on a beating heart. Furthermore, using stroboscopic illumination (1 flash per heart cycle) microvessels are always in focus and "appear" stationary. With this approach in the beating heart, there is little trauma to the coronary system and I can obtain continuous measurements of coronary microvascular pressures and diameters. The specific goals are to answer the following questions: (1) What is the distribution of resistance in the coronary circulation of the left ventricle? (2) What are the changes in the distribution of resistance in the left ventricular coronary circulation during autoregulation? (3) What are the effects of neural stimulation or norepinephrine infusion on the distribution of resistance in the coronary circulation of the left ventricle?